LIVING MATTER 



15 



evaporates. Where does it go ? As it becomes a gas, the 

 particles on the surface move away from their fellows 

 and pass into the nearest space that is open to receive 

 them. If the water is in contact with air, this space is 

 between the particles of the air. (Fig. 7, A). The air 



B 



Fig. 7.— A diagram illustrating diffusion. Two substances in contact, A, before; 

 B< after diffusion has begun. 



particles are also moving rapidly and they pass into the 

 spaces between the particles of the water. (Fig. 7, B). 

 The particles of each substance then continue to move 

 from space to space in the other until each holds as much 

 of the other as it can, or until one of them is entirely 

 taken up by the other. This process is called diffusion. 



Osmosis. — If the water were separated from the air 

 by a moist membrane, diffusion would take place 

 through the membrane. As the membrane is a solid, its 

 particles do not move away from each other, but through 

 the spaces between them the particles of air and of water 

 pass to the other side and the two substances become in- 

 termingled. (Fig. 8). This process is called osmosis. 



The Question Answered. — Through the process of 

 osmosis a liquid or a gas can pass through such a solid 



